Conventional AC bidirectional electric motors typically have three motor windings: the main "run" winding, the clockwise start winding, and the counter-clockwise start winding. To turn the motor on, AC voltage is first applied to the run winding. Soon thereafter, AC voltage is applied to one of the two start windings. This forces the motor to turn in one direction or the other. Once the motor is running, voltage can then be removed from the start winding. A unidirectional motor will only have a single start winding, since it only needs to be started in one direction.
When AC voltage is first applied to the run winding, the current is very high and the motor is not as inductive as while running. This is commonly known as the "locked-rotor" state, because the motor is not turning. When the motor does start to turn, the inductive characteristic increases and as a result, the current through the motor is reduced.
In conventional starter circuits, the start winding is usually connected in series with a solenoid. The solenoid is turned on by the high locked-rotor current of the motor, and the solenoid in turn provides power to the start winding. After the motor reaches running speed, the current through the solenoid is reduced sufficiently for the solenoid to drop out, thus removing voltage from the start winding. For bi-directional capability, two start windings are provided, with a relay or another switch determining which start winding is used, and therefore which direction the motor rotates.
The problem with these conventional approaches is that the cost of the solenoid is high, both in terms of its purchase price and in the additional labor costs and wiring harness expense that are required to install it into an end product.